The present invention relates to a lock-up control device and method for a lock-up type automatic transmission.
In automatic transmissions provided with a torque converter in the power transmission system thereof, the torque converter occasionally permits slippage between the input element thereof (pump impeller) and the output element thereof (turbine runner) and thus causes poor power transmission efficiency and bad fuel economy. Thus, some of the vehicles have adopted a lock-up type automatic transmission wherein the input element of the torque converter is directly connected to the output element thereof during operation of the vehicle in a relatively high vehicle speed operating range and low load operating range wherein the torque multiplication by the torque converter is not required and the variation in the torque of the engine does not create a problem.
A lock-up control device has been described in and in connection with FIG. 2 of copending U.S. application Ser. No. 308,595 entitled "LOCK-UP CONTROL SYSTEM FOR LOCK-UP TYPE AUTOMATIC TRANSMISSION," filed on Oct. 5, 1981, and commonly assigned herewith. In this lock-up control device, there is provided a vehicle speed sensor for generating a pulse train having a period variable depending upon the vehicle speed and a timer for generating clock pulses. A counter counts the number of clock pulses and generates a period indicative decimal signal indicative of the current period of the pulse train. The decimal signal represents the vehicle speed. A lock-up decision circuit including an arithmetic circuit and a memory receives this decimal signal indicative of the vehicle speed and generates a H level lock-up command signal when the vehicle speed is higher than a minimum lock-up vehicle speed which varies depending upon whichever gear ratio is used. When the vehicle speed drops and becomes lower than the minimum lock-up vehicle speed, the level of the lock-up command signal switches to L level. The lock-up command signal is fed to one of the two inputs of an AND gate. A driver is provided which, when the AND gate provides a H level signal, energizes a lock-up solenoid to cause a torque converter to lock up, whereas, when the AND gate provides a L level signal, ceases to energize the lock-up solenoid to release the torque converter from the lock-up state thereof. The level of the signal provided by the AND gate varies depending upon the level of the lock-up command signal as far as a H level signal is fed to the other input of the AND gate. The other input of the AND gate is connected to an Q terminal of a flip-flop which is set by the closure of an ignition switch and reset by a H level signal from an abnormality detecting circuit. This circuit maintains the H level of its output as long as the period of the pulse train does not exceed a predetermined time. The output of the circuit switches to a L level only when the period of the pulse train should become longer than the predetermined time, which occurs when the vehicle speed sensor ceases to operate. The flip-flop is set when the ignition switch is closed and keeps generating an H level signal as long as the vehicle speed sensor operates normally. However, when the vehicle speed sensor ceases its operation due to for example, a disconnection or short circuit, the flip-flop is reset and changes its output to L level. Therefore, inasmuch the L level signal is fed to the AND gate from the flip-flop, the AND gate keeps generating a L level signal, ceasing energization of the lock-up solenoid, thus releasing the torque converter from the lock-up state thereof.
With this control device, wherein when the period of the pulse train from the vehicle speed sensor increases and becomes longer than a predetermined period corresponding to the minimum lock-up vehicle speed, there is encountered a drawback that the actual release of the torque converter is delayed to cause the engine to stall when the vehicle is decelerated from the state wherein the torque converter locks up at such a rate as to invite the occurrence of a light wheel lock. The stalling of the engine possibly occurs when disconnection of the electric circuit or short circuit which causes the vehicle speed sensor to generate pulses having excessively long period.